Note: The minimum power sensitivity on most 802.11b clients is -96dBm (very low). If your software displays "Signal/Noise" or "SNR" in dBm, you can convert this to dB by subtracting the minimum power sensitivity, -96dBm, from the number displayed as "SNR" or "Signal/Noise". For example, if the Signal/Noise is -47dBm, you would convert this to dB as follows:
-47dBm - (-96dBm) = 49dB
SNR | Maximum data rate |
8dB | 5.5Mbps |
6dB | 2Mbps |
4dB | 1Mbps |
Possible problems
Other factors can affect the quality of your wireless connection. The list below is incomplete, but it may offer some explanation for poor performance that occurs even when the signal strength is good:- Multipath: In general, an RF signal grows wider as it is transmitted farther. As it spreads, the RF signal will meet objects in its path that will interfere with the signal in various ways (e.g., by reflecting it). When the signal is reflected by an object (e.g., a metal object) while moving toward a receiver, multiple wave fronts are created, one for each reflection point. This can result in a large number of waves, depending on how many reflecting surfaces the original signal encounters. Many of these reflected waves are still moving toward the receiver, creating a condition known as multipath. As the number of reflective surfaces increases, the signal deteriorates.
- Near/far: Near/far is a problem that can happen when multiple wireless users have devices that are very near an access point, much closer than a user who's on the radio signal boundary. The farthest device cannot be heard over the traffic from the devices closer to the access point. The only solution is to move the more distant device closer to the access point.
- Hidden node: When you turn on an 802.11b-capable laptop or handheld device, it immediately scans the airwaves for access points. It quickly evaluates the signal strength of the available access points, and the number of users per access point. Based on this, the device will choose the access point with the strongest RF signal and the fewest users. In hidden node situations, at least one client (node) is unable to "hear" one or more of the other clients connected to the same access point. Usually this is because of some physical obstruction between it and other users. As a result, there can be problems in the way the clients share the available bandwidth, causing data "collisions", or bit errors. When a bit error occurs, the clients need to re-transmit the data. These collisions can result in significantly degraded data transmission rates in the wireless network.
- Dynamic Rate Shifting (DRS): The terms Adaptive (or Automatic) Rate Selection (ARS) and Dynamic Rate Shifting (DRS) denote how bandwidth is adjusted dynamically by wireless clients. This adjustment in speed occurs as distance increases between the client and the access point (or possibly if interference increases). As the distance grows greater, the signal strength will decrease to a point where the current data rate cannot be maintained. As the signal strength drops, the client will drop its data rate to the next lower specified data rate, for example, from 11Mbps to 5.5Mbps, or from 2Mbps to 1Mbps.
Throughput
Throughput is a measure of the speed of your wireless connection. Defined as the amount of data transmitted in a given time period, throughput is based on many factors. Three important factors are described below:- Interference from another radio frequency source: IU uses the 802.11b wireless standard, which operates in a frequency range that is unlicensed, meaning the Federal Communications Commission allows anyone to use it. Unfortunately for 802.11b users, microwave ovens and 2.4GHz cordless phones operate in the same frequency band. The radio signal emanating from such devices can severely degrade or completely destroy an 802.11b signal.
- Security: 802.11b networks, if not secure, are susceptible to hacking and data theft by unauthorized users. Any sensible network owner will use some form of security. This adds more overhead to the wireless data packets; this overhead (bits of data) uses valuable bandwidth, but is absolutely necessary. Although 802.11b allows for 11Mbps maximum throughput, a user will typically get only about 5.5-6Mbps of data. IU uses a virtual private network (VPN) for encryption and authentication. Another method is wired equivalent privacy (WEP), but it is much less secure than VPN.
- Distance: Greater distances between the transmitter (access point) and receiver (client) will cause the throughput to decrease because of an increase in the number of errors (bit error rate). 802.11b recognizes these bit errors and requires that the bits be retransmitted. 802.11b is configured to make discrete jumps to specified data rates (11, 5.5, 2, and 1Mbps). If 11Mbps cannot be maintained because of bit errors and degrading signal strength, then the device will drop to 5.5Mbps, then to 2Mbps, and then to 1Mbps, with eventual loss of the connection. Remember, too, that because of overhead, you'll get only about half of the available bandwidth.
